1. Technical Field
The embodiments herein generally relate to analog television systems, and, more particularly, to a reliable SECAM line identification and PAL switch for analog television.
2. Description of the Related Art
Séquentiel couleur à mémoire (SECAM) (Sequential Color with Memory) is a standard for analog color television systems that is also compatible with monochrome television receivers predating its introduction. Because of this compatibility requirement, a second signal is added to the basic monochrome signal, and this signal carries the color information, called chrominance or C in short. SECAM uses frequency modulation (FM) to encode chrominance information on the sub carrier. In color television systems based on SECAM, blue color information and red color information are modulated with two separate color sub-carrier tones. Instead of transmitting the red and blue information together, it only sends one of them at a time, and uses the information about the other color from the preceding line. It uses an analog memory device called a delay line to store one line of color information.
The blue color information and red color information are transmitted in alternate lines. For example, the first line may transmit blue color information, the second line transmits red color information, the third blue, the fourth red, and so on. Alternatively, the order of the blue and red can be swapped. Blue color is modulated with a color sub-carrier tone at 4.25 MHz, while the red color is modulated with a tone at 4.40625 MHz. Also, for each line, there is a color reference sub-carrier tone along with the regular video signal. For a line with blue color information, the color reference sub-carrier tone of that line has the same frequency as the tone used to modulate the blue color information, which is 4.25 MHz. For a line with red color information, the color reference sub-carrier tone of that line has the same frequency as the tone used to modulate the red color information, which is 4.40625 MHz. To detect whether a line carries blue or red color information, one needs to detect whether the reference tone of that line is 4.25 MHz or 4.40625 MHz.
To decode the color information, an FM modulator is used. In an ideal case, for a pure reference tone, the output of the FM modulator will be a DC level. The level of the DC signal for a tone with a higher frequency will be higher than the level of the DC signal for a tone with a lower frequency. Thus, for SECAM, the DC signal corresponding to the red color line will be higher than corresponding to the blue color line. Based on this property, one can know whether a line carries blue color information or red color information.
The process used to decide whether a line carries red or blue color information is called SECAM line identification. In a noisy channel, the FM output corresponding to the reference tone could be very noisy and could result in bad line identification, thus leading to color distortion of the pictures.
Phase Alternating Line (PAL) is another standard for color television broadcasting. In this standard, the color information on the video signal is reversed with each line. In the receiver side, one must correct this reversed phase. The process of detecting and correcting the phase is called PAL switch. Traditional PAL switching techniques are based on a hard decision from the phase detector of the burst locked oscillator in the receiver. In the noisy channel, it is easy to suffer errors and leads to color shift and distortion.